Cypress CSC-1200T Guide de l'utilisateur télécharger pdf (Page 47)

Chapter 5. Fortran programming 47

a(j,i) = b(j,i) + 1

a(j,i+1) = b(j,i+1) + 1

END DO

Here we used the inverse operation of loop splitting to decrease the

overhead due to loop control.

Bottom loading is an eﬀective technique for overlapping loop control

and loading of operands for the next iteration of the loop. Here is an

example:

DOi=1,100

a(i) = a(i) + b(i)*c(i)

END DO

After each iteration, one has to check whether to do further iterations, or

to continue from the next statement after the loop. When the statement

a(i) = a(i) + b(i)*c(i)

is executed, one has to issue load operations for the a(i), b(i) and

c(i) values, which can take some time. Therefore, one could start the

load operations for the next iteration ﬁrst, and only after this check if

we should do another iteration.

Bottom loading can cause a program error if we try to load, for example,

the value c(101), which could be outside the memory allocated to the

program. In practice this never occurs, except in cases where the loop

has a large increment:

DOi=0,10000, 1000

a(i) = a(i) + b(i)*c(i)

END DO

Here we could load the value c(11000), which could be outside the

memory bounds.

The cache_align directive can be used to align arrays or COMMON blocks

on cache line boundaries:

REAL, DIMENSION(50) :: a, b

REAL, DIMENSION(10) :: c

COMMON /my_block/ a, b

!dir$ cache_align /my_block/, c

Here both the contents of the COMMON block my_block and the array c

were aligned on the cache line boundary. Therefore the array elements

a(1) and c(1) map to the ﬁrst word of a cache line.

The directives bounds and nobounds tell the compiler to check speciﬁc

array references for out-of-bounds errors:

!dir$ bounds [array_name [, array_name]...]

!dir$ nobounds [array_name [, array_name]...]

If the array names are not supplied, the directive applies to all arrays.

1 2 ... 42 43 44 45 46 47 48 49 50 51 52 ... 123 124

Pas de commentaire

Cypress CSC-1200T Guide de l'utilisateur Page 47